The requirements for high areal density recording impose increasingly greater requirements on thin film magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high density magnetic rigid disk medium for longitudinal and perpendicular recording.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Therefore, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al--Mg) alloy. Such Al--Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials have been employed, such as glasses, e.g., an amorphous glass, glass-ceramic materials which comprise a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks. The use of glass-based materials, such as glass-ceramic materials, is disclosed by Hoover et al., U.S. Pat. No. 5,273,834.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and typically comprises a non-magnetic substrate 10 having sequentially deposited on each side thereof an underlayer 11, 11', such as chromium (Cr) or a Cr-alloy, a magnetic layer 12, 12', typically comprising a cobalt (Co)-base alloy, and a protective overcoat 13, 13', typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 11, 11', magnetic layer 12, 12', and protective overcoat 13, 13', are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer. A conventional perpendicular recording disk medium is similar to the longitudinal recording medium depicted in FIG. 1, but does not comprise any underlayers.
Conventional methods for manufacturing a longitudinal magnetic recording medium with a glass or glass-ceramic substrate comprise applying a seed layer between the substrate and underlayer. Conventional methodology for manufacturing a perpendicular recording medium do not usually comprise applying any seed layer. Longitudinal magnetic recording media with glass or glass-ceramic substrates are commercially available from different manufacturers with different seed layer materials to reduce the effect of high thermal emissivity of such glass and glass-ceramic substrates, and to influence the crystallographic orientation of subsequently deposited underlayers and magnetic layers. Such conventional seed layer materials also include nickel-phosphorous (NiP) which is typically sputter deposited on the surface of the glass or glass-ceramic substrate at a thickness of about 500 .ANG.. Sputtered NiP films on glass or glass-ceramic substrates were reported in the literature for the control of crystallographic orientation of the longitudinal magnetic media and the enhancement of coercivity (for example, Hsiao-chu Tsai et al., "The Effects of Ni.sub.3 P-sublayer on the Properties of CoNiCr/Cr Media Using Different Substrates," IEEE Trans. on Magn., Vol. 28, p. 3093, 1992).
Conventional longitudinal magnetic recording media comprising a glass or glass-ceramic substrate having NiP sputtered thereon also comprise, sequentially deposited thereon, a Cr or Cr-alloy underlayer at an appropriate thickness, e.g., about 550 .ANG., a magnetic layer such as Co-Cr-platinum (Pt)-tantalum (Ta) at an appropriate thickness, e.g., about 350 .ANG., and a protective carbon overcoat at an appropriate thickness, e.g., about 150 .ANG.. Conventional Cr-alloy underlayers comprise vanadium (V) or titanium (Ti). Other conventional magnetic layers are CoCrTa, CoCrPtB, CoCrPt and CoNiCr. The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the glass or glass-ceramic substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional protective carbon overcoat is typically deposited in a mixture of argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 .ANG. thick.
Longitudinal magnetic films exhibiting a bicrystal cluster microstructure are expected to exhibit high coercivity, low noise and high remanent squareness. In co-pending application Ser. No. 08/586,571 filed on Jan. 16, 1996, now U.S. Pat. No. 5,830,584, issued Nov. 3, 1998 a magnetic recording medium is disclosed comprising a glass or glass-ceramic substrate and a magnetic layer exhibiting a bicrystal cluster microstructure. The formation of a bicrystal cluster microstructure is induced by oxidizing the surface of a seed layer so that the underlayer subsequently deposited thereon exhibits a (200) crystallographic orientation which, in turn, induces a bicrystal cluster microstructure in a magnetic alloy layer deposited and epitaxially grown on the underlayer.
U.S. Pat. No. 5,733,370 discloses a method of manufacturing a magnetic recording medium comprising a glass or glass-ceramic substrate and a magnetic layer exhibiting a bicrystal cluster microstructure. The disclosed method comprises sputter depositing an NiP seed layer on a glass or glass-ceramic substrate and subsequently oxidizing the deposited NiP seed layer. The oxidized upper seed layer surface induces the subsequently deposited underlayer to exhibit a (200) crystallographic orientation which, in turn, induces the magnetic alloy layer deposited and epitaxially grown on the underlayer to exhibit a bicrystal cluster microstructure. The magnetic recording media disclosed in co-pending application Ser. No. 08/586,571 now U.S. Pat. No. 5,830,584 and U.S. Pat. No. 5,733,370 exhibit high coercivity, low magnetic remanence (Mr).times. thickness (t) and low noise, thereby rendering them particularly suitable for longitudinal recording.
In copending application Ser. No. 09/152,324 filed on Sep. 14, 1998, now pending, the adhesion between a seed layer, particularly a NiP seed layer, and a non-conventional substrate, was improved by providing an adhesion enhancement layer, such as Cr or a Cr alloy, between the substrate and the seed layer, with an additional benefit in recording performance obtained by surface oxidizing the seed layer.
The entire disclosures of co-pending applications Ser. No. 08/586,571 now U.S. Pat. No. 5,830,584 and Ser. No. 09/152,324 now pending and U.S. Pat. No. 5,733,370, are incorporated by reference herein.
Some glasses and glass ceramic materials have lithium (Li) and sodium (Na) transition element additions to lower the glass transition temperature of the material. Lowering the glass transition temperature makes forming of glass products easier. A large amount of Li, e.g. about 0.5 to about 32 wt. % of Li.sub.2 O is incorporated into SiO.sub.2 matrix in ionic form and bonds in an ionic and secondary fashion in the SiO.sub.2 networks. The nature of the bonding enables leaching of the Li ions from the glass matrix. A typical magnetic recording medium comprises a CoCr alloy film as a recording layer. The media noise is mainly due to the exchange coupling between the CoCr alloy grains. In order to enhance the Cr segregation into CoCr alloy grain boundary to reduce the intergranular exchange coupling, high temperature sputtering is widely used in the magnetic rigid disc manufacturing industries. The typical substrate temperature during sputtering is about 200.degree. C. to about 250.degree. C. It typically takes several to more than ten minutes to sputter deposit the plurality of films in a pass-by in-line sputtering system. Because the melting point of pure Li is 181.degree. C., the driving force for Li diffusion in the process with so high temperature for so long a time is very large.
The media used in perpendicular magnetic recording do not usually comprise Cr alloy underlayers. Even for the media used in longitudinal magnetic recording, the Cr alloy underlayers can not seal the Li or prevent leaching.
It is well known that sputtered Cr and Cr alloy underlayers of thin film rigid discs exhibit an aggregate of faceted conical columns (Agarwal, S., "Structure and Morphology of RF Sputtered Carbon Overlayer Films," IEEE Trans., Magn., MAG-21, P. 1527, 1985.) The crystalline grain boundaries of the Cr and Cr alloy films are high-diffusion-rate paths. Therefore, the longitudinal magnetic recording rigid discs with Cr or Cr alloy underlayers directly deposited on Li-containing glass or glass-ceramic substrates and the perpendicular recording discs on glass or glass-ceramic substrates often suffer from Li corrosion problems. The Li leaching from the substrates further promotes Co leaching from the magnetic layers of the rigid magnetic discs, and makes the corrosion problems even worse. Corrosion products will be picked up by the recording head causing smearing on the recording head and disc surface, resulting in increased stiction and eventual drive failure.
There exists a need for technology enabling the use of glass and glass-ceramic substrates containing large amounts of Li in magnetic recording media while preventing Li migration from the substrate.